1. Field of the Invention
The present invention pertains to fluid energy conversion systems and, more particularly, to energy conversion systems having variable, load-matching transformers for maximizing energy supplied to a load by a fluid energy source or conversion device, such as a wind turbine or a water turbine, where the fluid energy source and/or load vary with time.
2. Discussion of the Prior Art
Wind and water turbines operate at optimum efficiency only at a specific shaft torque which changes with variations in the fluid energy source. Optimum turbine rotation speed, determined by load torque and by the fluid energy source, will also vary with time. Turbine shaft torque will reflect variations in the energy-receiving load, e.g., changing voltage and/or frequency, changing water head, or changing gas pressures. To maximize energy or power transfer under changing source and load conditions normally requires a continuously-varying transformer which causes the torque reflected from the load to the turbine shaft to equal the instantaneous optimum torque for the fluid energy source.
There is much literature concerning torque-matching of a turbine to a variable fluid energy source. The energy-receiving load is often regulated to be substantially constant with time, e.g. utility lines or near-constant voltage batteries. Consequently, little attention has been given to matching variable loads; however, the following examples show that load variation with time is significant for several important wind-power applications.
First, in a wind-driven water pump used to fill a reservoir, hydraulic head, the pump's load, varies with water height in the storage reservoir and with water table depth, a function of rainfall and pumping-dependent drawdown. Hydrostatic head variation in a shallow-well with a highly rainfall-dependent water table, or a storage reservoir with considerable fill depth, is as great as two-to-one, or greater.
Second, in a compressed air energy system with storage tanks, tank pressure will vary over time as stored energy fluctuates. Back-torque from a fixed-geometry, positive-displacement compressor will vary little with changing compressor shaft speed but will vary significantly with changing reservoir pressure.
Third, in a refrigerant gas compressor driven by a wind or water turbine to pump heat from a varying ambient-temperature source into a variable-temperature heat reservoir, refrigerant gas pressures are determined by temperatures and temperature-dependent vapor-pressures in the evaporator and condenser, and pressures will commonly vary by a factor of two or more on both sides of the compressor, causing torque of a fixed-geometry compressor to vary by a factor of four under usual operating conditions.
The above examples present difficulties in efficiently matching variations in the fluid energy source, even if the load is presumed constant. Considering a wind turbine, the most economical wind turbines operate efficiently at high tipspeed ratios, i.e., high ratios of turbine-tip tangential velocity relative to windspeed upstream of the turbine's disturbing influence. Such high-speed turbines depend on high tangential velocity of the blades to develop large aerodynamic forces resulting in torque. In a given wind, turbine starting torque may be only 20% as great as torque at optimum-power speed, or starting torque may be zero. A high tipspeed-ratio turbine may be unable to start a positive-displacement pump or compressor under load, even under the load torque that would be optimum for energy transfer if the turbine were started and operating. For starting, some device must intervene in the power transmission path, such as a clutch, a variable-ratio rotary transmission, or a compression relief valve.
Once spinning, the turbine should operate at a constant tipspeed ratio to maintain a constant advance angle of the turbine blade tip and, thus, maintain all parts of the turbine blades at their most efficient angles relative to the fluid flow. For constant tipspeed ratio, rotation speed varies linearly with wind-speed. Since dynamic pressures (or Bernoulli pressures) vary as the square of windspeed, torque will vary as the square of both rotation speed and windspeed at constant tipspeed ratio. Power, the product of torque and rotation speed, will consequently vary as the cube or rotation speed and windspeed under optimum loading conditions. Accordingly, there is a great need for variable load-matching power or energy transformers for use with wind or water powered turbines. As a guideline to the needed range of variability, cube-law wind energy becomes negligibly small below roughly eight MPH. For most sites, windspeeds in excess of eighteen MPH are quite infrequent, such that the cost of designing a Wind Energy Conversion System (WECS) to operate at top efficiency for some windspeed above eighteen MPH is seldom economically justified in terms of average energy payback. A WECS that operates in a twelve MPH average wind regime, that begins power conversion at eight MPH, that is optimally efficient from eight to eighteen MPH, and that governs at constant power for winds exceeding eighteen MPH, will recover about 66% as much energy as a similar hypothetical (but impractical) system with optimum efficiency in all windspeeds. 30% of the remaining energy represents high-end governing loss and only 4% represents the remaining low-end energy loss. Recovery of the lost 34% is usually not economically worthwhile because of design costs. The variation from eight to eighteen MPH is a speed ratio of 2.25, representing a square-law torque ratio of roughly 5 and a cube-law power ratio of roughly 11. Wind systems designed for higher or lower average wind regimes will generally require about the same ratio of torque and power variation for full turbine/load compensation.
The matching problem for a water turbine is similar to the wind turbine case. When hydrostatic head is converted to velocity of a water jet hitting a turbine, velocity varies as the square-root of head pressure. Optimum turbine torque varies as the square of both rotation velocity and water velocity, therefore linearly with hydrostatic head. Because of blade strength and cavitation limitations, water turbines do not operate at high tipspeed ratios. Consequently, starting torque of water turbines is relatively high, unlike many wind turbine situations. Besides operating with variable head, many water turbines use a variable-width nozzle to regulate flow to the turbine according to energy demand and water supply variations. Optimum turbine torque with a variable nozzle should vary as the product of nozzle orifice area times hydrostatic head or, equivalently, as the product of orifice area times velocity squared. Optimum turbine speed and tipspeed ratio is barely affected by nozzle orifice size. Power varies as orifice area times velocity cubed, or as orifice area times head to the 1.5 power.
Water turbines are excellent candidates for water pumping (to provide needed head for irrigation, municipal water supply, etc.), gas compression, and especially refrigerant compression for heat pumping, air-conditioning and refrigeration. The water used to drive the turbine is an excellent source and sink for thermal energy.
Most wind electric generators operate without specific turbine-to-load matching compensation. Average load mismatch losses for battery-chargine and resistance heater loads are less than 5%, provided the system is optimized for average load match. With constant-speed alternators and near-constant-speed induction generators operating into fixed-frequency utility grids, the best compromise constant turbine speed represents a 5% to 10% loss of recoverable power. A variable-displacement hydraulic transmission has been used to permit a range of constant-tipspeed-ratio turbine operation while the synchronous alternator operates at constant RPM. Most large alternator systems adjust field current with power lever to optimize power factor and minimize losses as power varies. It appears that little or no work has been done with time-varying electrical loads except for using battery storage which absorps the variations and presents a wind generator with a relatively fixed voltage.
Adaptive load matching to lift water or compress gases (including for refrigeration) is much more critical. A high torque, low-tipspeed-ratio multibladed turbine driving a single-acting piston water pump with buoyant shaft (to avoid even worse starting load) will recover less than 30% of potentially available wind energy (not including losses when the turbine furls or turns out of the wind in high winds). Double-acting pumps and multicylinder compressors spread the load more evenly through the rotation and can recover, ungoverned, about 45% to 60% of recoverable power, although the figure drops drastically if a cheaper high-speed turbine is used. Turbines with automatic clutches can perform reasonably well, recovering up to 65% of available power before accounting for governing, which lowers the figure.
Thus, it will be appreciated that there exists a great need for energy conversion systems capable of maximizing energy transfer from a fluid energy source, such as a wind or water turbine, to a load, and prior art attempts to maximize such energy transfer have not been effective even with the great amount of effort directed thereto.